The present invention relates to a power MOSFET circuit for switching on and off an inductive load, and more specifically to a so-called high side switching circuit. The circuit according to the present invention may be an integrated circuit or a circuit of discrete components.
One conventional high side switching circuit employs a vertical power MOSFET fabricated by the double diffusion technique (as disclosed in Japanese Patent Provisional Publication No. 59-98558). The drain of the power MOSFET is connected with a power supply, and the source is grounded through an inductive load.
In this high side switching circuit, a counter electromotive force is produced across the inductive load when the power MOSFET turns off to switch off the inductive load. Because of this counter electromotive force, the source potential of the power MOSFET becomes negative and lower than the ground potential. As a result, the gate potential of the power MOSFET becomes higher than the source potential, and the power MOSFET turns on again. This undesired on/off phenomenon continues while decaying until the current is reduced almost to zero by an internal resistance of the circuit. Therefore, the switching operation of the power MOSFET from the ON state to the OFF state requires a considerable time determined by a time constant of a loop circuit formed by the power source, power MOSFET and the inductive load. This conventional switching circuit is unsatisfactory because of its slower switching speed especially when it is used for switching a load of a valve in a hydraulic system of an automobile where a high speed switching-off operation is required.
Furthermore, this conventional switching circuit is susceptible to surge. The inductive load tends to receive adverse influences of a nearby current flow due to induction, and a counter electromotive force can be readily produced between both ends of the inductive load by a surge. If such a counter electromotive force is produced while the inductive load is in the OFF state, then the source potential of the power MOSFET becomes lower than the ground potential (=the gate potential), and accordingly the power MOSFET is erroneously turned on and causes a malfunction of the inductive load.
To overcome these problems, a flyback diode is used in another conventional circuit. The diode is connected between both ends of the inductive load , and the anode of the diode is grounded. The cathode of the diode is connected to a branch point between the inductive load and the source of the power MOSFET. During normal operation, the source potential of the power MOSFET is equal to or higher than the ground potential, so that the flyback diode is reverse biased and held in the OFF state. If a counter electromotive force is produced across the inductive load, the source potential of the power MOSFET decreases below the ground potential, and the flyback diode is put in the forward bias condition. In this condition, the flyback diode permits current flow in a loop formed by the diode and the inductive load and by so doing, holds the source potential of the power MOSFET at the level of the ground potential to prevent an undesired turnon of the power MOSFET.
In this conventional configuration, however, the flyback diode must handle a heavy current flow in the load, so that the current-carrying capacity of the diode must be sufficiently high. Furthermore, an equivalent impedance (R) of the loop is low since the flyback diode is forward biased. Therefore, a time constant .tau. (=L/R) describing the decay of current in the loop of the inductive load (L) and the diode becomes greater, and the switching-off time of the inductive load becomes longer.